This application is based on Application No. 2001-139731, filed in Japan on May 10, 2001, the contents of which are hereby incorporated by reference.
1. Field of the Invention
The present invention relates to a brush assembly incorporated in a motorized power steering apparatus for example.
2. Description of the Related Art
FIG. 6 is a cross sectional side view of a known motorized power steering apparatus, and FIG. 7 is a front elevation of a brush assembly 40 incorporated in the motorized power steering apparatus.
This motorized power steering apparatus includes an electric motor 30 for generating a rotating force or torque and an electromagnetic clutch 2 connected with the electric motor 30 for controlling the transmission of the rotating force or torque thereof.
The electric motor 30 includes: a cylindrical yoke 3; four field permanent magnets 4 housed in and fixedly secured to the yoke 3 in an opposed relation with respect to each other; a shaft 7 rotatably supported by a first bearing 5 and a second bearing 6 in the yoke 3; an armature 8 fixedly mounted on the shaft 7; a commutator 9 having a plurality of segments 9a (see FIG. 10) fixed to the shaft 7 near one end thereof; a brush assembly 40 having brushes 32, which are urged into abutting engagement with a surface of the commutator 9 under the action of elastic forces of corresponding springs 10, and brush holders 31 holding the corresponding brushes 32, respectively; a housing 14 to which the brush holder 31 is fixedly attached through a magnetic shield plate 41, the housing being made of a non-magnetic material in the form of aluminum and connected with the yoke 3 by means of fastening screws 19, and a grommet 16 through which a line or conductor 15 extends. The magnetic shield plate 41 comprises a flat plate which is made, for example, by punching a rolled steel sheet of a thickness of 1 mm by press working to form a doughnut-shaped configuration.
The armature 8 is provided with a core 17 which has a plurality of slots extending in an axial direction, and a winding 18 which is formed of a conductor fitted in and wound around the slots of the core 17 in a plurality of turns.
The electromagnetic clutch 2 includes a clutch stator 20 fixed to the housing 14 by clutch fastening screws 21, a clutch coil 22 disposed in the clutch stator 20, a boss 23 rotatably mounted on one end of the shaft 7 through a third bearing 6a, a drive rotor 24 fixedly mounted on the shaft 7, and a doughnut-shaped disk 25 fixedly secured to the boss 23 through a spring member 26.
The brush assembly 40 includes a base 35 with a plurality of conductors 36 embedded therein by insert molding, four metal brush holders 31 fixed to the base 35, four brushes 32 held in the corresponding brush holders 31 and being urged to abut against the commutator 9 under the action of the elastic forces of the corresponding springs 10, and a plurality of lead wires 33 each having one end thereof connected with a corresponding brush 32 and the other end thereof connected with a connecting portion 34 of a corresponding conductor 36. The brushes 32 and the connecting portions 34 of the conductors 36 are arranged in line symmetry with respect to center lines A and B, which extend in diametrical directions passing through the center of the shaft 7, and which are perpendicular with respect to each other.
In the motorized power steering apparatus as constructed above, current is supplied to the winding 18 through the brushes 32 which are in abutment with the segments 9a of the commutator 9, so that the armature 8 is driven to rotate together with the shaft 7 under the action of an electromagnetic force.
On the other hand, by energizing the clutch coil 22, a magnetic circuit MC is formed by the clutch stator 20, the drive rotor 24 and the disk 25, all of which are made of magnetic materials. The spring member 26 is caused to flex or deform toward the clutch stator 20 under the action of the magnetic circuit MC thus formed, whereby the disk 25 is magnetically attracted and fixed to the drive rotor 24, which is thereby made integral with the boss 23.
Thus, the rotating force of the shaft 7 is transmitted to the drive rotor 24, which is fixed to the shaft 7, and thence to the boss 23, from which the rotating force is further transmitted to a warm shaft (not shown) splined with the boss 23, thereby assisting the steering force of a steering wheel of a vehicle (not shown).
FIG. 8 and FIG. 9 are explanatory views for explaining the conductivity orientation of each brush 32 of the brush assembly 40.
Electroconductive particles 41 (for instance, copper powder) contained in each brush 32 are turned into the powder particles (for instance, copper powder) deformed into tabular or flattened configurations in a direction perpendicular to the direction of pressing by a compressive stress generated upon press molding of each brush 32. Therefore, an orientation is created in the electroconductivity of each brush 32. A Z direction of each brush 32 in FIG. 9 is the direction of the press molding (i.e., pressing direction). The electric resistance of each brush 32 in the Z direction indicated by arrow Z exhibits a value greater than that in an X direction indicated by arrow X (i.e., in the direction of rotation of the shaft 7), or in a Y direction indicated by arrow Y (i.e., in a diametrical direction of the shaft 9). Generally, the ratio of the Z-direction resistance to the X-direction or Y-direction resistance is about 4 to 5.
FIG. 10 is a view illustrating the appearance in which a brush 32 press-molded in the Z direction is in abutment against the commutator 9. In this figure, the brush 32 is formed at its opposite edges with tapered contact portions 32a and 32b extending in parallel with the direction of rotation of the commutator 9, the tapered contact portions 32a and 32b being in abutment with the surface of a segment 9a of the commutator 9. A tip end of a lead wire 33 is embedded in the corresponding brush 32 in such a manner that it extends up to about the middle (i.e., half) of the length L of the brush 32.
In the brush assembly 40 of the above construction, when a comparison is made between current paths, indicated at arrows m0 and m2, respectively, passing through the brush 32 in FIG. 10, the current path indicated at arrow m2 is inclined in a direction of arrow Z in comparison with the current path indicated at arrow m0, and hence has an accordingly larger electric resistance, so that there takes place a greater temperature rise in the brush 32 when the electric motor 30 is energized. As a result, for example, the base 35 made of synthetic resin or plastic would be melted, causing a defective sliding of the brush 32.
Though in relatively infrequent cases such as, for example, when a car with the motorized power steering apparatus is put into a garage, large current is supplied to the electric motor 30 thereby to abnormally raise the temperature of the brush 32. Thus, there arises a problem in that certain measures have to be taken in order to prevent such situations. That is, for example, a maximum current to be supplied to the electric motor 30 or the time during which current is continuously supplied to the electric motor 30 is limited; or the sizes or dimensions of the respective component parts such as the brushes 32, etc., of the brush assembly 40 are enlarged in order to increase the thermal capacity and the effect of heat radiation.
The present invention is intended to obviate the problems as referred to above, and has for its object to provide a brush assembly in which electric power losses in brushes can be reduced to effectively suppress an abnormal rise in temperature of the brushes and hence the entire brush assembly.
Bearing the above object in mind, according to one aspect of the present invention, there is provided a brush assembly comprising: a brush having three or more tapered contact portions which are in contact with a surface of a commutator fixedly secured to a shaft; and a lead wire having a tip end portion thereof embedded in the brush. The brush is press-molded in a pressing direction perpendicular to a normal of the surface of the commutator. The contact portions of the brush are disposed at opposite edges of the brush in the pressing direction and at an intermediate portion between the opposite edges. The tip end portion of the lead wire is inserted into the brush in the pressing direction so as to extend up to a location or farther therefrom corresponding to a second one of the contact portions counted from a lead wire inserting side of the brush.
According to another aspect of the present invention, there is provided a brush assembly comprising: a brush having two tapered contact portions which are in contact with a surface of a commutator fixedly secured to a shaft; and a lead wire having a tip end portion thereof embedded in the brush. The brush is press-molded in a pressing direction perpendicular to a normal of the surface of the commutator. The contact portions of the brush are disposed at opposite edges of the brush in the pressing direction. The tip end portion of the lead wire is inserted into the brush in the pressing direction so as to extend over two thirds or more of the length of the brush in the pressing direction.
The above and other objects, features and advantages of the present invention will become more readily apparent to those skilled in the art from the following detailed description of preferred embodiments of the present invention taken in conjunction with the accompanying drawings.